AbstractMicrosomal material from lactating bovine mammary tissue possess very active acyl‐CoA:1‐acyl‐sn‐glycerol‐3‐phosphorylcholine acyl transferase (EC2.3.1.‐1). Oleyl‐CoA was the preferred substrate and rates of 6.3, 6.4, 1.8, 21.0, and 0.1 nmoles acylated/min/mg were obtained for myristyl‐, palmityl‐, stearyl‐, oleyl‐ and linoleyl‐CoA, respectively, when 1‐oleyl‐sn‐glycerol‐3‐phosphorycholine was used compared to 5.2, 4.1, 1.0, 3.5, and 0.2 nmoles/min/mg with 1‐palmityl‐sn‐glycerol‐3‐phosphorylcholine as acyl acceptor. Apparent Km values of 4.5 and 5.2 μM for oleyl‐CoA and 6.5 and 4.6 μM for palmityl‐CoA were obtained with 1‐oleyl‐ and 1‐palmityl‐sn‐glycerol‐3‐phosphorylcholine, respectively. The Km values of 1‐oleyl‐sn‐glycerol‐3‐phosphorylcholine were 20 and 50 μM using oleyl‐CoA and palmityl‐CoA as acyl donors. The possible involvement of this enzyme in membrane dynamics during lipid secretion is discussed.